The present invention relates to a cellulose hydrate-based food casing and its use as a sausage casing.
Flat, in particular web-like and sheet-like, and tubular cellulose hydrate-based food casings are conventionally manufactured by the viscose process. In the viscose process, an alkaline cellulose xanthogenate solution termed a "viscose solution" is extruded through a ring or slot-shaped die, coagulated in acid solution as cellulose hydrate gel and regenerated to form cellulose hydrate.
By changing the viscose composition and incorporating additives, the properties of the food casing can be varied. These modifications can be performed both on reinforced and also on unreinforced casings. Unreinforced cellulose hydrate casings are termed transparent cellulose films and are known as Cellophane.RTM.. Fiber-reinforced food casings are obtained by coating one or both sides of a web-like or tubular fiber framework with a viscose solution, impregnating it and then treating with a coagulation solution and regeneration solution. Such casings are particularly used for naturally-ripened and mold-ripened long-keeping sausage.
Food casings fulfill important functions in the manufacture, ripening and storage of sausage. They must be supple, sufficiently extensible and storable for long periods.
The suppleness and processability of casings are conventionally improved by secondary softeners, such as glycerol, glycol, or polyglycol. The secondary softeners are not covalently bound to the cellulose hydrate, but are only bound by intermolecular forces. When the casing is soaked in water prior to filling or when the sausage is scalded or cooked, the secondary softeners are virtually completely dissolved out. This causes shrinkage, compaction, and embrittlement of the casing after drying the sausage. Owing to the shrinkage, the internal pressure of the sausage can increase so much that it bursts. These phenomena are due to a crystallization of the cellulose hydrate.
In order to decrease the crystallization tendency, so-called "primary" softeners, which effect a permanent softening, are added to the viscose solution. The primary softeners used are compounds which react with the cellulose molecules, such as N-(C.sub.9 -C.sub.24)-alkyl-N,N',N'-tris(hydroxymethyl)urea or similar compounds that have long aliphatic carbon chains. See U.S. Pat. No. 4,002,712. However, unreactive compounds, such as di-and polyhydroxy compounds which are esterified with long-chain aliphatic monocarboxylic acids (See DE-A 26 54 427 equivalent to U.S. Pat. No. 4,253,879), or alkylene oxide-based polymers having at least one N-hydroxymethyl-carbamate group (See DE-A 26 54 417 equivalent to U.S. Pat. No. 4,198,325) are also used.
However, the water permeability of the sausage casings is barely influenced by these softening methods. As low a water permeability as possible of the cellulose casings is important for the ripening behavior and mold growth in the case of naturally-ripened and mold-ripened, long-keeping sausage. The even ripening of long-keeping sausage demands slow release of water through the casing during the first days.
Fiber-reinforced cellulose casings exhibit a high water permeability of generally about 90 to 110 l of water per square meter of casing in 24 hours at a pressure of 40 bar. Sausages manufactured with these casings are therefore ripened in ripening chambers in which a uniformly high relative air humidity can be maintained. Insufficient or varying air humidity leads to so-called dry edges on the sausage. The reason is excessively rapid drying out of the sausage filling surface in the first days of ripening. The dry edge decreases further water loss, so that the interior of the sausage is still moist even after the conventional ripening time.
In order to decrease the water permeability, attempts have been made to compact the cellulose hydrate casing, for example, by repeated drying of the casings or by the inclusion of cross-linking agents, such as cyclic ureamethylol. See DE-A 22 46 829 equivalent to U.S. Pat. No. 3,937,672.
Collagen casings, i.e., casings made of cured protein, have the desired swelling index and the low water permeability which is generally only about 10 l of water per square meter in 24 hours at a pressure of 40 bar. However, mechanical strength, toughness and caliber constancy which are essential criteria for the quality of sausage casings leave much to be desired in collagen casings.
A further quality characteristic of cellulose hydrate casings is their moisture balance. Thus, the casings should be able to absorb as much water as possible, i.e., have a high swelling index. To determine this swelling index, the casing is softened in water and adhering water is centrifuged off at a defined speed of rotation. After weighing, the casing material is completely dried and again weighed. The weight difference, expressed in percent of the weight of the dry casing gives the swelling index. A swelling index of 120% therefore means that a casing of 100 g dry weight absorbs 120 g of water. The casings should release the water again only slowly.
The water permeability gives no indication of the amount of water which can be absorbed or released. The permeation is solely a measure of the water permeability of the casing, whereas the moisture balance characterizes the water absorption and water retention capacity. A good moisture balance ensures a problem-free drying process in which no embrittlement or overdrying of the casing occurs.
By the addition of alginic acid or salts thereof to the viscose solution, a permanently softened sausage casing having an improved swelling index can be obtained. See DE-A 40 02 083. These sausage casings manage even without glycerol as a secondary softener. Alginic acid or alginate loosens up the structure of the cellulose hydrate, therefore the degree of compaction of the glycerol-free cellulose hydrate is highest with lower alginic acid or alginate concentration, the minimum concentration being 5% by weight, based on the weight of the cellulose. However, these casings are paper-like and wrinkle in the dry state or at low moisture contents. This disadvantage can be diminished by increasing the alginic acid/alginate concentration. However, in this case the permeation increases, which leads to impaired ripening properties of the sausages.